The contrasting effects of nitrogen and phosphorus fertilizations on the growth of Cunninghamia lanceolata depend on the season in subtropical China

Abstract

Phosphorus (P) and nitrogen (N) are the major limiting nutrients for plant growth in most ecosystems. However, it is unclear whether P fertilization exhibits higher effects on plant growth than N fertilization and whether the effects vary with season, since plant demands for N vs. P vary differently during different seasons. We seasonally measured N and P concentrations of fine roots, leaves, and soil as well as growth in Chinese fir (Cunninghamia lanceolata) plantations fertilized with P (5 g m−2 a−1) and/or N (5 g m−2 a−1 and 10 g m−2 a−1) over five years in subtropical China. Needle N and P concentrations were significantly influenced by P fertilization, whereas root N and P concentrations were significantly influenced by P fertilization and the interaction between P fertilization and season. Phosphorus fertilization generally decreased root N concentration in the warm season and increased P concentrations of needle and root in the cold season. Moreover, root P concentration with P fertilization was significantly higher during the cold season than during the warm season, regardless of N fertilization. Contrary to the cold season, in the case of high N fertilization, basal area increment was significantly lower with than without P fertilization during the warm season. Nitrogen fertilization in the warm season had a positive effect on basal area increment, contrary to the cold season. Contrary to N fertilization, P fertilization had a negative effect on basal area increment in the warm season, whereas it had a positive effect in the cold season. Phosphorus fertilization contributed more than N fertilization to basal area increment during both seasons. Moreover, root P concentration was the most crucial factor determining basal area increment during both seasons. The opposite effects of N and P fertilizations on plant growth depend on the season in the Chinese fir plantation, which suggested that the timing of P and N fertilizations should be considered for the Chinese fir plantations in subtropical areas.